Calorimeters are devices for measuring the heat absorbed or released by a chemical reaction. Automated devices available today perform appropriate measurements and utilize those measurements to determine automatically instantaneous rates of heat exchange. These rates can be summed or integrated to provide a net or effective heat of reaction.
Swiss Patent No. 455,320 discloses an automatic calorimeter in which a heat transfer fluid is circulated at a constant flow rate through the interior of a reaction vessel by means of a coil. The temperature of the liquid is varied in response to the difference between a predetermined set-point temperature and an interior temperature of the reaction vessel. The temperature variation of the fluid is several times greater than the actual deviation between set point and sensed reaction vessel temperatures to anticipate the delay in the response of the system. The remainder of the heat transfer fluid circulation system is essentially closed and includes a heating and/or cooling device in a loop with the reaction vessel. Fluid passing through the coil is returned to the heating/cooling device and its temperature is adjusted in response to the difference between the set point temperature and the reaction temperature.
U.S. Pat. Nos. 3,994,164 and 4,456,389 disclose successive improvements to the device of Swiss Patent No. 455,325. The devices of the two U.S. patents differ from that of the Swiss Patent by incorporating simultaneously operating heating and cooling systems and controllers which vary the outputs of the two systems to vary the temperature of the fluid entering the reaction vessel. Additionally, each of the reaction vessels includes a mantle or jacket surrounding an inner shell containing the reaction mass for circulating fluid around the shell rather than through a coil within the shell.
One of the major drawbacks of the prior art calorimeter devices referred to above is that they require potentially widely diverging temperature swings of the heat transfer fluid. Since heat transfer characteristics (i.e. specific heat) of the fluid vary with temperature, more error is introduced by increasing the temperature range to which the heat transfer fluid is subjected.
Another significant drawback of the Swiss device is that system response to unanticipated rapid exothermic or endothermic reactions would be slow because of thermal inertia of the heat exchange fluid. It would be difficult to quickly cool down or heat up the heat transfer fluid, the heating device reservoir and piping to maintain or rapidly bring the reaction under control. Failure to respond quickly to a reaction exotherm could result in a runaway reaction and possible explosion.
The calorimeter devices of U.S. Pat. Nos. 3,994,164 and 4,456,389 provide two reservoirs for more rapid response. However, these devices circulate the heat transfer fluid through a jacket surrounding the reaction chamber shell. Jacketed vessels have a more limited heat transfer surface than can be achieved with a coil immersed in the reaction mass. Reaction vessels are typically made of glass which is a poor heat transmitter. These factors can lead to drift of the reaction vessel interior temperature from the desired set point, ultimately resulting in inconsistent results and/or runaway reactions.
Moreover, in such jacketed systems, when the temperature of the reaction mass approaches that of the jacket, a large degree of error can be introduced when measuring a heat of reaction if the jacket temperature varies significantly from ambient temperature. This is due to heat transfer between the jacket and surrounding atmosphere.
Lastly, conducting the heat transfer fluid through a jacket surrounding the sides of the reaction vessel will obscure the only good view an operator may have of the reacting mass, even if a glass walled vessel is employed.
The method of operation of all of these prior art devices tends to magnify certain system errors. As is pointed out in the Swiss patent, heat transfer is related to the temperature difference (.DELTA.T) between the heat exchanger inlet and outlet temperatures. As U.S. Pat. No. 3,994,164 points out, the system is operated so as to keep the temperature difference between the heat exchanger inlet and outlet to less than one degree Centigrade. These temperatures are measured by instruments which have a limited accuracy. As the temperature difference being measured becomes smaller, the percentage contribution of instrumentation erro to the measurement becomes greater.
It would be beneficial to provide a calorimeter and method of operating such device which rapidly responds to reaction mass temperature variations.
It further would be beneficial to provide a calorimeter and method of operating such device in which the temperature excursions of the heat transfer fluid are kept to a minimum to minimize any errors introduced due to variations in the specific heat of fluid.
It further would be beneficial to provide a calorimeter and method of operating such device in which temperature differences between the heat transfer fluid inlet and outlet from the reaction vessel are significantly more than one degree Centigrade to minimize the contribution of temperature measurement errors in the determination of heat transfer.